1. Field of the Invention
This invention relates to a high speed dot printer for use as an output device for a data processing system such as a computer, a word processor and a drafting machine.
2. Description of the Prior Art
Impact serial dot printers have been known in which while a print head having a plurality of print wires equally spaced in a direction of a print sheet feed and having a plurality of electromagnetic solenoids for driving the print wires is reciprocated in a direction perpendicular to the print sheet feed, the plurality of print wires are selectively driven to impact the print sheet through an inked ribbon in accordance with printing information so as to cause the printing of characters, patterns and such. Also, in order to increase print speeds, impact dot line printers have been proposed in which while a print head having a plurality of print wires equally spaced in a direction perpendicular to a print sheet feed and having a plurality of electromagnetic solenoids for driving the print wires is continuously reciprocated in a direction also perpendicular to the print sheet feed, the plurality of print wires are selectively driven to impact the print sheet through an inked ribbon so as to cause the printing of characters, patterns and such.
These impact printers allow printing by impacting mechanically print wires or the like onto a print sheet. On the other hand, as serial dot printers or dot line printers like the aforementioned impact printers, there are also available, in various fields, non-impact printers such as thermal printers which allow printing non-mechanically on a special print sheet utilizing Joule's heat produced by application of a pulse current to a heating resistor, and ink jet printers which jet ink on a print sheet utilizing a momentary pressure wave generated by strain of a piezoelectric-crystal element.
In these serial dot printers and dot line printers, a bidirectional printing method has been used for effecting printing during both forward and return movement of the print head. However, in these printers, when printing characters or figure patterns requiring to drive successively printing elements such as electromagnetic solenoids and heating resistors on a thermal head, there is the possibility that a current supplied to the printing elements and their driver circuits will be increased and exceed the maximum rated current of the power system. In the past, therefore, in order to protect power systems when an output current exceeds the maximum rated value, they are designed to have a characteristic that the voltage abruptly drops (droop) or another characteristic that both voltage and current are reduced. However, if the output of the power system becomes low, printing pressure will run short, which may cause characters to appear unclear or not to be printed. In this case, it is necessary to interrupt printing.